Changes In Corneal Biomechanics During Small Incision Lenticule Extraction (SMILE) And Femtosecond-assisted Laser In Situ Keratomileusis (FS-LASIK)

Background:Myopia is the most common refractive error in the world.Corneal refractive surgery has become the most popular surgical method for correction of myopia in adults in the past decade.Corneal ectasia is a very serious complication after corneal refractive surgery.It also directly affects the visual quality after corneal refractive surgery.Corneal biomechanics has been proved to play an important role in the progress of corneal ectasia.Corneal refractive surgery changes corneal biomechanics and corneal structure.There is a close relationship between corneal refractive surgery and corneal biomechanics.At present,the most commonly used corneal refractive surgery methods are: SMILE and femtosecond FS-LASIK.There is no definite conclusion as to which of the two surgical metho ds has greater impact on corneal biomechanics.Both of them involve femtosecond laser cutting:SMILE creating corneal lenticule,FS-LASIK creating corneal flaps.There is no report on the biomechanical effects of femtosecond laser cutting of these two surgi cal methods.Corvis Scheimpflug technology(Corvis ST),as a tool for in vivo analysis of corneal deformation parameters,has been developed to the second generation(Corvis ST Ⅱ).Some new parameters need to be further explored.Purpose:Observe the influence of femtosecond laser cutting on corneal biomechanics during SMILE or FS-LASIK and assess the biomechanical changes following the operation.And observe the changes of biomechanically corrected intraocular pressure(b IOP),and evaluate the accuracy of b IOP in clinical measurement of intraocular pressureMethods:Prospective,non-randomized study.A total of 80 eyes from 80 patients were treated with either SMILE or FS-LASIK.Parameters of inverse concave radius,deformation amplitude ratio 2 mm or 1 mm(DA ratio 2 mm or 1 mm),highest concavity radius(HC radius),biomechanically corrected intraocular pressure(b IOP),and central corneal thickness(CCT)measured by Corvis ST Ⅱ were recorded at 1 day preoperatively,immediately after the lenticule or flap creation,during subsequent lenticule extraction or excimer laser ablation,and during follow-up at 1 week,1 month and 3 months postoperatively.All statistical analyses were performed using SPSS19.0.Results:After lenticule creation,the DA ratio 2 mm or 1 mm were bigger(p<0.05)meanwhile CCT was thicker(p<0.05)than with flap creation.Partial parameters changed significantly after lenticule creation or flap creation,and all parameters changed significantly after tissue removal.All parameters showed no significant differences between the two groups(p>0.05)after operation.The variations in b IOP(Δb IOP)after operation were significantly less than those in Δnon-contact IOP(p<0.001)in two groups.Conclusions:Femtosecond laser cutting during lenticule creation has a greater impact on corneal biomechanics than flap creation.Both the femtosecond laser cutting and removal of tissue degrade corneal biomechanics;however,these effects may be predominantly a result of tissue removal.SMILE and FS-LASIK have no differences in corneal biomechanics when the same CCT is consumed.b IOP is more reliable after the operation,but further study is essential.